Lamp bulbs, forming the actual light source, of high-pressure discharge lamps, are apt to become very hot. The light source base portion itself should be retained in a base structure which carries connecting terminals, such as terminal blades, pins or the like. Voltages arising in high-pressure discharge lamps, particularly metal halide discharge lamps, are high. Compact versions of metal halide discharge lamps, having power outputs in the order of about 100 W, frequently require ignition or firing while the lamps are still hot. Such lamps of 100 W power, usually, are single-ended, that is, are single-based with base terminals at one end portion of the lamp. Hot re-ignition requires use of a suitable ignition circuit which applies short-duration pulses in the order of about 20 kV. The exact value of ignition pulse voltage depends on the power rating and design of the lamp.
The high-voltage pulses required for hot re-ignition may result in spurious sparks or flash-overs within the base. The dielectric strength of air is about 1 kV per millimeter. The spacing between current supply leads in the base of compact metal halide discharge lamps is small, however, and frequently in the order of about only 1 cm. Spurious discharges or flash-overs often start as creep discharges or creep arcs between the lamp connecting terminals, directly along the lamp bulb. The material of the lamp bulb or lamp vessel may, itself, increase the tendency to permit flash-over.